So who's running the union? Some measure of nonsensicality exists here in the idea of a union composed entirely of warehouse workers that is somehow magically acting against the best interests of those warehouse workers.
...is this hard to believe? This happens all the time with unions. Professional sports unions fall for this all the time, most notably Major League Baseball and the National Football League. The former butchered their negotiations so bad in the last Collective Bargaining Agreement (CBA) that they screwed themselves out of hundreds of millions of dollars due to their own ineptitude and the latter settled a concussion lawsuit that didn't require ownership to admit that they negligently and criminally lied about the safety (or lack thereof) of union members at their job.
Unions regularly act against their own interests; rarely do they have legal/arbitration representation nearly as good as ownership, and this is only one factor that causes it. Oftentimes they have leadership that prioritizes the needs of the few (seniority) over the needs of the many, or are just outright corrupt.
Collective decision making and fiduciary duties are directly contrary to each other but unions are supposed to feature both. The way the circle is squared is that unions don’t in fact represent all their members’ interests, only those of 50% + 1.
If only! In reality, long-living unions create such a layered bureaucratic structure that they don't represent most workers in most matters anymore (case in point: unions in Italy, since decades ago when they were protected by law.)
Hint: it pays off both mortgages and bar tabs.
There can be multiple unions too, and inter-union conflict. Some of the unions are very right wing (hate gays and women basically) and have e.g. opposed paid maternity leave in a zero sum game approach.